1. Field of the Invention
The present invention relates to an electrically conducting organic compound and, more specifically, the present invention relates to an organic semiconductor which can be used in the production of electronic devices such as transistors, particularly to an organic semiconductor applicable to flexible electronic devices such as electronic paper. The present invention also relates to electronic devices using this organic semiconductor.
2. Description of the Related Art
In the field of electronic display, liquid crystal displays and organic EL displays are improving in their performance and also making rapid progress toward higher precision and larger size. On the other hand, a display showing excellent portability by having, like paper, both good visibility and flexibility, namely, a flexible property capable of changing a shape by fold-bending, a so-called electronic paper, is keenly demanded. To realise this electronic paper, it is essential that a thin film transistor can be formed on a plastic substrate or, in other words, a flexible circuit for driving a picture element can be realized. However, currently existing electronic circuits mainly comprising an inorganic material such as polycrystalline silicon or amorphous silicon require a large-scale process such as high temperature and high vacuum and, taking account of the heat resistance and high production cost of the plastic substrate, these circuits are applicable, at best, to only a part of an instrument and cannot be used widely in practice. In order to solve these problems, attention is being focused on an organic semiconductor having excellent flexibility, requiring no high-temperature/vacuum process, such as vapor deposition, and allowing an inexpensive printing means to be applied.
The means for the film formation of an organic semiconductor are roughly classified into vacuum process, such as vapor deposition, and spin coating, casting or printing from a solution. In the case of applying an organic semiconductor to a device such as field effect transistor, the vapor deposition method capable of ensuring good crystallinity in molecule has been heretofore predominantly used so as to achieve as high a carrier mobility as possible. As for representative organic semiconductors formed by the vapor deposition method, oligothiophene (see, H. Akimichi et al., Applied Physics Letters, 58(14), Apr. 8, 1991), pentacene (see, C. D. Dimitrakopoulos et al., Applied Physics Letters, 80(4), Aug. 15, 1996), copper phthalocyanine and the like have been reported. On the other hand, as for representative organic semiconductors formed by casting, spin coating or printing from a solution, polythienylene vinylene (see, H. Fuchigami et al., Applied Physics Letters, 63(10), Sep. 6, 1993), polyalkylthiophene (see, A. Tsumura et al., Applied Physics Letters, Vol. 49, P. 1210 (1986), and Journal American Chemical Society, vol. 117, p. 233 (1995)) and the like have been reported. In addition, a film formation example where the LB (Langmuir-Blodgett) method is applied for forming a single molecular film or controlling the orientation property of molecule with an attempt to bring out a new function, has been also reported (see, J. Paloheimo et al., Applied Physics Letters, 56(12), Mar. 19, 1990). The method for calculating the field effect mobility of these molecules is described in detail in these publications.
Incidentally, in the case of using such an organic semiconductor as the channel layer of a field effect transistor, its field effect mobility is approximately from 0.1 to 0.01 cm2/Vs and this is about several figures lower than that of an amorphous silicon semiconductor (up to 1 cm2/Vs) even in a vapor deposition system which is reported to give a high mobility. In the case of forming the film from a solution, the field effect mobility is usually further lower, by about 1 or 2 figures, due to the difficulty in controlling the molecular orientation. In other words, a most important problem in realizing an organic semiconductor is how high field effect mobility can be achieved by a simple and easy production process. To have high mobility, the organic semiconductor generally must have a π-conjugate system largely extended within the molecule and furthermore, the organic semiconductor molecule must be oriented in the conductive direction. The oligothiophene, polythiophene and poly(p-phenylene vinylene) referred to above are a linear electrically conducting polymer where the π-conjugate system runs along the main chain. On the other hand, the material system where the π-conjugate system is planarly extended includes condensed polycyclic aromatic molecules such as ovalene, coronene and bianthrene. With respect to conventionally known means for controlling the orientation of organic semiconductor molecules, Japanese Unexamined Patent Publication (Kokai) No. 9-83040 discloses a technique of coating a π-conjugate polymer, such as polythiophene, on an orientation film substrate subjected to a rubbing treatment, or orienting the molecules on an orientation film by introducing a liquid crystalline substituent as a side chain or by applying an external force such as magnetic field or electric field.
However, the orientation force on an organic semiconductor by the orientation film is weak and, at present, a sufficiently high molecular orientation cannot be attained. The introduction of a crystalline substituent incurs a problem such that the conduction path of the charge carrier is decreased due to the presence of the substituent not contributing to the electrical conductivity and the mobility is rather reduced. The organic semiconductor where the π-conjugate system is planarly extended, such as pentacene, phthalocyanine and ovalene, has almost no solubility in a solvent and since the film is formed by vapor deposition, the cost is a great problem.